When synthesis gas is produced from carbon-bearing fuel material, the solids obtained normally must be removed from the process. The solids are, for instance, ash and slag, which as a rule are left in the form of lumps and thus cause clogging of the piping, valves or lock-type facilities. DE 3144266 A1 describes such a process, in which the ash and slag obtained by a gasification system are collected in a water bath; the latter is also called slag water bath. The ash and slag particles are batchwise removed by gravity flow from the gasification system by means of a lock-type transfer vessel fitted underneath the said system. In this case, lock-off devices are mounted upstream and downstream of the lock-type transfer vessel so that the said vessel is separated on the fluid side from the gasification system. When the lock-type transfer vessel is filled with slag it is under elevated pressure, too, because it is connected to the gasifier. In order to preclude any blocking of the upper shut-off devices, a downward water stream containing particles is generated and flows across the shut-off devices. This is effected by withdrawing water from the upper section of the lock-type transfer vessel, preferably using a deflector sheet to separate the stream to be discharged in such a manner that only a minor part of particles is entrained by the stream to be discharged.
DE 60031875 T2 deals with a process for slag removal, the slag being obtained by the production of synthesis gas. In this case, a further intermediate vessel is arranged between the gasification device and the lock-type transfer vessel. As a part stream of water with a low particle content is withdrawn from the lock-type transfer vessel, a surge is produced so that the solids are removed from the intermediate vessel and enter the lock-type transfer vessel, thus avoiding any formation of bridging clusters of slag particles. The lock-type transfer vessel must therefore be sized such that the solids can freely settle. In an ideal configuration of the intermediate vessel, devices are mounted so as to provide a part stream of water with low slag content, too, which also improves the settling of slag particles from the gasification system in the intermediate vessel.
EP 0290087 A2 describes a solution suitable for the removal of slag deposits on and clogging of the shut-off devices arranged above the lock-type transfer vessel, i.e. a gas volume is created within the lock-type transfer vessel and subjected to a pressure lower than that of the gasification system. When the lock-type transfer vessel is connected to the gasification system by opening the upper shut-off devices, the difference in pressure initially generates a downward surge impact of water and slag such that any blocking above or upon the upper shut-off devices is eliminated. In this case, the gas volume is arranged in a circular space of the upper section of the lock-type transfer vessel, the said space being formed by the vessel shell and a pipe reaching into the said vessel.
DE 102008005704.5 describes a process for slag removal during the synthesis gas production. The slag is discharged from the coal gasification reactor and sent to a slag vessel with a liquid, which normally is water. The slag vessel is enclosed by the pressure vessel. A lock-type transfer vessel is mounted underneath the slag vessel in direction of gravity and separated from the slag vessel by means of a valve. This method allows a decrease in pressure of the slag flowing into a collecting vessel. A stream of liquid is sent to the circular space formed by the internals. Thus, a downward part stream of the cooling water coming from the slag vessel and containing some slag flows into the lower part of the lock-type transfer vessel, in a counter-current stream to the downward slag movement. In order to enhance the cooling effect, a constriction-type channel is formed by the respective internals such that it is possible to adjust cooling down to a value well under 100° C. and to avoid the formation of vapors during the depressurization of the lock-type transfer vessel. Moreover, a gas volume is arranged in the circular space at a pressure above that of the pressure vessel so that the connection of the lock-type transfer vessel with the pressure vessel causes a backward surge impact required to remove any formation of bridging slag clusters.
DE 102006040077 A1 also describes a process for the removal of slag formed during synthesis gas production. The slag is discharged from the coal gasification reactor and sent to a slag vessel filled with a liquid. A lock-type transfer vessel is arranged in direction of gravity underneath the slag vessel and separated from the latter by means of a valve to discharge the slag. A part stream of liquid is withdrawn from the lock-type transfer vessel and sent to the pressure vessel in order to remove any deposits or blockage from this area. DE 102006040077 A1 shows that the liquid stream is withdrawn at a point of the vessel with a low slag concentration to preclude any entraining of larger slag particles. The cooling water fed to the lower section of the collecting vessel makes a portion of cooling water ascending across the collecting vessel such that the bulky slag is loosened and that the required cooling is achieved before the expansion vessel. Cooling of the slag and water inventory in the vessel is necessary to avoid a formation of vapors during depressurization. The period required for this task depends, inter alia, upon the volume of slag and water in the lock-type transfer vessel.
The processes described above exhibit essential disadvantages. The provisions made for the avoidance of operational trouble during the slag discharge and for loosening blockages require a large dead inventory in the water-filled vessel which consequently cannot be exploited for slag bulking. The dead volume obtained in the described processes may be as large as 50% of the total inventory. The oversize required for the vessels involved causes additional costs for making the lock-type transfer vessels and a large space requirement for integrating them into the plant equipment. Furthermore, the large water inventory in relation to the quantity of slag in fact constitutes a real load for the downstream plant units. In addition, the operational flexibility of the plants is restricted because the dead volume saturated with water must likewise be cooled. This requires additional time and causes prolonged cycle intervals of the lock-type transfer vessel. As a matter of fact, the processes described above merely achieve an undefined separation of coarse and fine particles during the discharge of the liquid stream from the lock-type transfer vessel. Coarser particles unintentionally entrained by the discharged liquid stream may entail an increased erosive load for the downstream equipment, such as piping and pumps, and in the worst case this may cause a shutdown of the complete plant. Furthermore it is not desired to perform a simultaneous discharge of smaller particles still bearing a portion of carbon and of the coarser slag particles. On the contrary, it is common practice to remove the fine particles from the liquid stream in a separate filtration or separation step and to recycle them into the process, if any.
Therefore, an objective of the present invention is to provide a process and device that are suited for an undisturbed removal of slag obtained by the synthesis gas production and to minimize the accumulation of non-useful slag volumes in the lock-type transfer vessel and achieve a high accuracy of separation of fine and coarse particles.